1. Field of the Invention
The present invention relates to a printing rubber blanket used in lithography, and more specifically to a printing rubber blanket with which sinking down of the rubber blanket, which occurs while using the blanket, can be reduced, and the reduction of thickness of the rubber blanket is less, thereby achieving an excellent durability.
2. Description of the Related Art
The lithographic offset press printing operates in the following manner. First, the printing rubber blanket rotates while being brought into tight contact with the plate cylinder on which characters and images are formed and printing ink is provided, and thus the ink of the characters and images on the plate cylinder is transferred onto the rubber blanket. Then, the characters and images on the rubber blanket are (transferred and) set on a print medium such as paper sheet, which is conveyed as being brought into tight contact with the rubber blanket, thereby carrying out printing.
The rubber blanket, which is conventionally employed, includes a smooth surface rubber layer having a thickness of about 0.4 mm, a fabric layer underlying the rubber layer to integrally adhered to the surface rubber layer, a compression layer underlying the fabric layer to integrally adhered to the fabric layer, and two to four fabric layers that are stacked one on another via adhesive layers alternately under the compression layer. The total thickness of the printing rubber blanket is in a range of 1.16 to 3 mm.
As shown in FIG. 3, the printing rubber blanket is wound around the cylinder at a high tension. FIG. 3 illustrates a rubber blanket 1, a blanket cylinder 2, which is a cylinder around which the rubber blanket is wound, and a bar member 3 mounted on an end portion of the rubber blanket 1.
The lithographic offset press that uses such a rubber blanket as described above, applies a very high printing pressure between the plate cylinder and the rubber blanket and also a very high transferring pressure between the rubber blanket and the impression cylinder in order to obtain a print that has no uneven density on its printed surface but has an excellent reappearance of halftone.
Therefore, in actual printing, the rubber blanket used here undergoes severe dynamic shock repeatedly. As a result, the rubber blanket, as it is used, loses its thickness, which is the phenomenon called “sink down”. As the rubber blanket sinks down and the thickness of the rubber blanket reduces, the printing pressure acting between the plate cylinder and the rubber blanket naturally reduces.
As the reduction in the printing pressure between the plate cylinder and the rubber blanket occurs, the transfer of the ink from the plate cylinder is not properly performed, and the transfer of the ink is not sufficiently carried out. Especially, in the case where the rubber blanket is used under severe conditions such as in high-speed printing, the sink-down of the rubber blanket is further promoted, and therefore it is likely that the reduction in the thickness of the rubber blanket occurs in an early stage. Under these circumference, the life of the rubber blanket is significantly shortened in the case of high-speed printing, at present.
It is well known that one of the main factors of the sink-down of a rubber blanket is the reduction in thickness of the woven fabric of the fabric layer of the blanket. In order to suppress the sink-down of the rubber blanket, thereby decreasing the reduction in the thickness of the rubber blanket, the following technique is conventionally known. That is, the woven fabric of the fabric layer used for the rubber blanket is in advance subjected to calendering, in which fabric are allowed to pass between calendar rolls to be crimped, and thus the thickness of the woven fabric is reduced in advance.
FIG. 7 shows the calendering of woven fabric 4, which serves as the fabric. FIG. 7 illustrates a pair of calendar rolls 5 and 6. The pair of calendar rolls 5 and 6 may be a pair of a metal roll and a metal roll, or a metal roll and a resin roll. The woven fabric 4 is passed through the gap between the calendar rolls 5 and 6, thereby compressing by pressing the woven fabric 4 in advance. A rubber blanket that uses a highly dense woven fabric prepared by the compression, as its fabric layer, can reduce the sink-down.
In rubber blankets, there is a close relationship between the degree of the sink-down of the fabric and the density of the woven fabric of the fabric layer.
It is known that the woven fabric applied to a rubber blanket exhibits a larger sink-down degree as the density of the woven fabric is lower, after repetitious compression when the rubber blanket that uses the fabric is actually used. Therefore, the calendering of the woven fabric carried out in advance before it is used for the rubber blanket, to increase its density, brings a remarkable effect in the reduction of the sink-down of the rubber blanket. Thus, the technique of calendering the woven fabric used in a rubber blanket to make it highly dense, thereby suppressing the sink-down of the rubber blanket, has been widely employed.
However, rubber blankets that include a fabric layer that uses a calendered woven fabric have such properties that as the time passes, the woven fabric recovers its thickness before it was calendered. Thus, a rubber blanket that uses a calendered woven fabric cannot maintain its thickness stably for a long period of time, but the woven fabric recovers its original thickness. In this manner, it becomes easy for the rubber blanket to have the sink-down, which creates a problem.
Rubber blankets that employ a calendered woven fabric of, particularly, cotton fiber, polynosic fiber, rayon or mixture of these fibers, are sensitive to temperature and moisture and tend to recover their thickness to the thickness before the calendering. Thus, there are possibilities that rubber blankets which easily create sink-down are formed, which is a serious drawback.
The lithographic offset press uses dampening water during printing at all times. The damping water easily permeates inside the rubber blanket from the edge portion thereof during printing, and the water eventually will reach the fabric layer of the rubber blanket. When a portion of the damping water reaches the fabric layer of the rubber blanket, the fabric layer easily swells as it absorbs the water portion, and the portion of the fabric layer easily recovers the thickness before being calendered.
In the state described above, the rubber blanket regionally creates a difference in thickness between its edge portion and the other section. When such a difference is created, the printing pressure and transferring pressure are not constantly applied by the rubber blanket, but a difference is created between the edge portion and the other portion. Due to this drawback, there is conventionally a possibility of occurrence of printing troubles including uneven printing. Under these circumstances, there has been a demand for an invention of rubber blanket which rarely reduces its thickness by sink-down or rarely creates a regional difference in thickness, even after it is used in operation for a long period.